Introduction

Wheels are one thing, what about making something a bit more complicated?

Bob Armstrong is making a PDP–8 thing. Its really cool. Since I too
am a fan of older computers, I offered to help in what ever way I
could. Well the switches he found for the front panel all have white
switch handles. We needed some burnt orange and yellow ones. Seemed like
something I could do with casting plastic.

This is the target:

Two Views of the Source Material

Side View

Bottom View

You can’t tell by the image, but the switch fits entirely within the
outline of a US Quarter, so its not very big. The good news is that
Alumilite picks up detail but the small space leads to some interesting
challenges.

I started by trying to create a mold for the switch handles. Unlike the
wheels, this could not be a pour mold. Instead it had to be something that
had two parts. The other thing I discovered was that you really do want
to pull a vacuum on your molds when they are small and complicated. Here
is a picture of making the bottom half:

Bottom Half Mold for the Switch

This mold shows the “paddle” of the switch sticking out of the RTV. The
theory was that I could bend the RTV to release the two “nubs” that the
switch uses to stay attached to the switch body.

I pulled a vacuum on this mold by using a FoodSaver machine, they are
sold here at CostCo (and CostCo Online) for about $140. The good news
is that you’re spouse will love that you gave them this surprise gift
that keeps leftovers fresh, lets you can your own foods, and generally
helps the environment. Just remember to save one of the canisters for
yourself :-). The above mold was vacuumed by using the ‘lid saver’
attachment (didn’t have the canister yet) and is shown below on the right,
A larger picture with the whole setup is shown below on the left. Basically this
vacuum pump will pull a vacuum in any bottle or jar (and there are
special bags as well but I don’t use those.)

Pulling a Vacuum on the Mold

Larger View of the Setup

Closeup of the Setup

The goal of course is to get all of the air bubbles out of the RTV before
it sets. This is especially important when you’re making a mold that has
a lot of detail and sharp edges. I wanted the “nubs” on the switch to
come through crisply as that way they would work in the switches they
were intended for.

Once the bottom was cast, it was time to cast the top. This is where the
first challenge comes in. If you’re using RTV in your mold and more RTV
for the top, you have to keep the two from sticking together. That is
done with “mold release” powder. (basically its just talcum powder). I
prepped the mold with mold release and was ready to do the top half.

Bottom mold with Mold Release Prep

The trick was that the top half would have the top edge of the paddle
right at the surface. After the mold was created I would use the
rectangular hole as a fill hole for the mold.

If you have a two part mold you have to fill it somehow, and the resulting
fill “appendage” where the plastic hardens in the fill hole is called the
“sprue.” I thought I would avoid a sprue by using the end of the switch
as the fill hole. So more RTV is mixed, the top is filled, and put under
vacuum once again.

I don’t have pictures but the resulting two part mold was nearly
impossible to pull apart. That meant I had not used enough mold release. I
did get them apart and another problem became obvious, the vacuum causes
the RTV to surge briefly as the air is being purged, and that caused it
to overflow my mold holder, then when the RTV sank back down the switch
end was sticking out. So much for using it as a sprueless mold! Not to
be completely dissuaded, and since the casting part is the “fast” part,
I used the mold I had made and cast a switch handle with it, the results
are below.

Somewhat broken two part mold

The “rough” surface of the two mold halves shows the problem with the
mold release, the RTV had bonded with itself and so the separation was
not clean. The resulting cast switch captured the body “form” really
well, and the interior is quite good too, but the nubs on the sides of
the switch did not do well at all. In fact the portion of the mold that
should have molded them ripped off during the separation process.

This mold is the most challenging casting thing I’ve done so far. Pour
molds are childs play by comparison. The next attempt is going to try
creating a displacement mold. In that version the mold will not have
a sprue hole, rather the switch impression will be filled with plastic
and the top half will insert a mold that will displace the plastic that
makes up the interior. Given the setting speed of this stuff I expect
it to be a challenge.